An unprecedented load on internet servers from people wanting to watch webcasts of the transit of Venus yesterday caught web managers unawares, some having to bring on board new supercomputers to deal with the load.

"It really stretched us to the limit," he said. "We didn't crash but we were running at 100% capacity."

Perth Observatory tried to spread the load by sending half their images to a supercomputer elsewhere but even that was not enough.

"We had to transfer to another server because we were crashing our ISP," observatory spokesperson Peter Birch told ABC Science Online. "It just took off. I didn't expect anything like we got.

"This was unprecendented."

Simon Dixon network administrator of the observatory's ISP, Highway 1, said the event put "a bit of load" on the system because it was unexpected.

"I didn't know about it until the server was under load," he told ABC Science Online.

He estimated there had probably been between one and a few million hits on the Perth Observatory site and he would prefer to be warned before something like this happened again.

Birch agreed: "In the future thought I think I would have a long discussion with the ISP first," he said. "I don't think they would have believed a transit of Venus would have generated millions of hits."

Not a new problem

The problem of webcasts overloading servers is not new. White cited the case of one webcast of a solar eclipse he was involved in during the late 1990s.

"I know that afternoon we stuffed up the university server. It was over 97% of its capacity for three hours," he said. "I got a funny letter from the web manager the next day saying 'Don't ever do that again'. He had his tongue in his cheek but I think he was very, very surprised at how successful it was"

White said problems were also evident during the 2002 total solar eclipse.

"The world was bristling with webcasts but I couldn't get on to any," he said. "I sat there for an hour trying to find just one site.

"I know the technology is good. I know people are interested. The real problem is that the web is just not big enough and fast enough to do the job."

CSIRO's Osborne said as internet capacity increased, so had demand.

"In 1995 it was not uncommon to expect a simple page to take a minute to download," he said. "Nowadays we expect full video and motion.

"It raises some interesting questions about webcasting," he said. "How do you do it and still satisfy everyone? It's not like TV where it doesn't matter if there's one or one million people watching."

As for the show itself, it seems clouds obscured some of the transit for those viewing webcasts from Perth and Canberra.

"We had a huge band of cloud move in just before the transit began," said Osborne. But things turned out in the end.

"Just before sunset a gap in the clouds appeared and for the next 15 minutes we watched the sunset with Venus in front of it."

Cloud also meant that Perth got "about half" the event.

No cloud at all from Townsville, though: "It was absolutely perfect," said White.

Osborne said it was unclear whether anyone has seen the "black drop" effect, in which the shape of Venus appears to distort as it leaves the face of the Sun. He said some theories suggested this could be because optics had improved since the last time the transit was observed 122 years ago.